Various interferometer applications sense scattering field, so the interference signal comprises the coherent contribution of many scattering speckles, which in general are random in phase and magnitude. Examples include coherent LIDAR ranging, Interferometry Doppler sensing and optical coherence tomography (OCT). In these examples, a single-point or single line-of-sight sensing can be naturally extended to a higher dimensional imaging application, by incorporating lateral beam scanning such as a raster scanning pattern. In imaging applications, the beam scanning range is typically large enough to cover more than ten resolvable beam spots and more than ten interferometer measurements are made. A standard beam scanning mechanism shifts a sensing beam continuously, resulting in a continuously changing scattering field during an interferometer signal acquisition period. Two image degradation issues may arise when a sensing beam moves continuously: spatial resolution and interferometer measurement precision.
The spatial resolution degradation is easy to understand. For example, if a circular beam spot shifts at a constant speed of one beam diameter per signal acquisition period, the resulted spatially resolvable spot becomes an oval shape, approximately two beam diameters in the scanning direction and one beam diameter in the orthogonal direction. The spatial resolution is worsened along the beam scanning direction and the asymmetry of the resolvable spot is often undesirable as well.
Continuous beam scanning introduces an arbitrary phase transition of a sensed speckle field during a signal acquisition period, which consequently increases interferometer measurement errors. Static speckle noise has been observed and studied extensively. For example, in a coherent LIDAR, the precision of distance measurements is typically limited by static speckle noise which is comparable to the surface roughness of a sensed target. The study of scanning speckle error is a relatively new. When the optical sensing beam remains stationary to the scattering field during a signal acquisition period, the scanning speckle error is zero. Baumann et al. published in “Comb-calibrated frequency-modulated continuous-wave ladar for absolute distance measurements,” Optics Letters 38, (2013), and proved that the scanning speckle error of a frequency modulated continuous-wave (FMCW) was statistically proportional to a dimensionless lateral beam scanning speed. The dimensionless scanning speed could be defined as the beam displacement during an interferometer signal acquisition period per resolvable beam width along the scanning direction. In a typical imaging application, it is reasonable to set the dimensionless scanning speed in the range of 0.2 to 2, and consequently, a scanning speckle error can be often found to be a dominating error source. Baumann did not teach any beam scanning methods to reduce the scanning speckle error in imaging applications. Thorpe disclosed a dual-chirped-laser coherence LIDAR in US Patent Publication Number US 2016/0123720 A1, that the scanning speckle range error could be reduced or removed by sharing a common sensing beam path with two chirped laser beams, and the wavelength chirping could be in a same direction or in opposite directions. However, this dual-chirped-laser technique is not capable of reducing a Doppler error caused by scanning speckle. Additionally, the increased complexity of signal processing of the dual-chirped-laser technique can be undesirable. For instance, the sensor data refreshing rate can be compromised if the computation power is limited. Thorpe mentioned that optical phase-sensitive detection techniques such as low-coherence interferometry, OCT and laser radar could face unique challenges in measuring diffusely scattering surfaces due to speckle. Thorpe's this statement did not teach beyond the familiar scope of static interferometer speckle. Neither Baumann nor Thorpe evaluated the effect of scanning speckle error on interferometers other than FMCW LIDAR measuring distance or displacement. Even for the dual-chirped laser LIDAR, which is capable of measuring Doppler shift in additional to distance, Thorpe did not mention the Doppler error caused by scanning speckle. It can be unobvious to extend the teaching of scanning speckle error in LIDAR distance measurement to some other interferometer measurements or some other types of interferometers.
Michie disclosed a step-scan weather radar in U.S. Pat. No. 5,392,048. Such a radar could certainly be an interferometer type. Acquiring interferometer data over a certain period, such an interferometer or another interferometer in general could make one measurement such as distance, or a set of measurements including, e.g., distance, velocity and signal strength. The step-scan mode could be automatic raster scan with continuous beam scanning in one direction and incremental beam steps between continuous scanning lines. The beam positioning could be operated as a selective mode to skip areas between regions of interest. Essentially, the interferometer could make a series of interferometer measurement in one area to form a set of sub-images while the beam continuously moved, and the interferometer could step to a next area to acquire another set of sub-images. Although continuous beam scanning can have obvious advantages with fewer interruptions, Michie taught that a disrupted scanning mode can have unique advantages worth the trade-off. However, Michie did not teach a repetitive step scanning mode allowing an interferometer to acquire signal under stationary beam condition. For common interferometers producing effective and efficient measurements, it can be much more complicate or difficult to operate such a step-scanning mode than run a continuous scanning mode. Furthermore, it can be extremely challenging to speed up such a step-scanning mode to 1 kHz or higher. A step-scanning operation obviously needs a strong incentive to offset the disadvantages. Improving on the previously described spatial resolution degradation might not serve as a strong incentive.
Relating to Michie's teaching, an interferometer can certainly idle at one sensing beam position and later move to a different position. A person can possibly teach an interferometer operation like this: holding a sensing beam stationary relative to a target at a first beam position while acquiring signal for two or more sets of interferometer measurements; moving the beam to a second position to acquire interferometer signal for one or more interferometer measurement sets and so on so forth. Although data averaging at the first beam position has little effects on static speckle noise, data averaging could reduce some other noise such as instrument noise. Even without knowing how relevant a scanning speckle error is, this person might also point out the absence of scanning speckle error because of the stationary beam. However, it is not obvious to extend this teaching to an effective and efficient imaging strategy. For instance, if the step size between the first position and the second portion is either smaller than 0.2 or bigger than 2 times the resolvable beam width, the beam sampling can be viewed as inefficient or ineffective for image forming. Also, it might not be viewed a meaningful imaging strategy to sample fewer than 10 beam positions or cover a range less than 10 resoluble beam widths. In addition, acquiring redundant periods of data at the first beam position can be undesirable for low efficiency in term of time expense. Similarly, it can be inefficient to make only one set of interferometer measurements at the first beam position, while two or more sets of measurements can potentially be made with the same quality. It can be either redundant or inefficient to spend twice or longer time at one beam position than at another beam position. Although it can be necessary to have a beam transition period between two beam positions, it can be inefficient to have a transition time longer than the signal acquisition time at either beam position. Therefore, many instrument operation modes can be distinctive from an effective and efficient step-scanning interferometer imaging strategy.
Ngoi pointed out in U.S. Pat. No. 6,271,924 that speckle noise was a significant noise source in Doppler vibrometer measurement. Ngoi also suggested that by adjusting the speed of continuous scanning, the measurement precision could be affected. Ngoi's definition of “scanning speed” lacked an important variable of sensing beam width at a target plane. Nevertheless, inspired by Ngoi's teaching, one might be able to experiment vibrometer precision as a function of “scanning speed” or become aware of a precision advantage at an as low as possible “scanning speed”. However, there are still two problems at a practical low continuous scanning speed: 1, oversampling pays for a heavy penalty of reducing image refreshing rate, and 2, the scanning speckle error can be significant even at a low scanning speed. Ngoi's vibrometer used an acoustic-optical-deflector (AOD) beam scanner. An AOD is capable of moving and stopping a beam within typically 1 μs, potentially offering a temporal window with a stationary beam and then quickly opening another window at a different beam location. Ngoi did not teach a step-scanning and measurement strategy for imaging an extended area.